Membrane insertion and topology of the translocon-associated protein (TRAP) gamma subunit.

Translocon-associated protein (TRAP) complex is intimately associated with the ER translocon for the insertion or translocation of newly synthesised proteins in eukaryotic cells. The TRAP complex is comprised of three single-spanning and one multiple-spanning subunits. We have investigated the membrane insertion and topology of the multiple-spanning TRAP-γ subunit by glycosylation mapping and green fluorescent protein fusions both in vitro and in cell cultures. Results demonstrate that TRAP-γ has four transmembrane (TM) segments, an Nt/Ct cytosolic orientation and that the less hydrophobic TM segment inserts efficiently into the membrane only in the cellular context of full-length protein.

A transmembrane serine residue in the Rot1 protein is essential for yeast cell viability.

Polar residues are present in TM (transmembrane) helices and may influence the folding or association of membrane proteins. In the present study, we use an in vivo approach to analyse the functional and structural roles for amino acids in membrane-spanning motifs using the Rot1 (reversal of Tor2 lethality 1) protein as a model. Rot1 is an essential membrane protein in Saccharomyces cerevisiae and it contains a single TM domain. An alanine insertion scanning analysis of this TM helix revealed that the integrity of the central domain is essential for protein function. We identified a critical serine residue inside the helix that plays an essential role in maintaining cell viability in S. cerevisiae. Replacement of the serine residue at position 250 with a broad variety of amino acids did not affect protein targeting and location, but completely disrupted protein function causing cell death. Interestingly, substitution of the serine residue by threonine resulted in sustained cell viability, demonstrating that the hydroxy group of the TM serine side chain plays a critical role in protein function. The results of the present study indicate that Rot1 needs the TM Ser250 to interact with other membrane components and exert its functional role, avoiding exposure of the serine hydrogen-bonding group at the lipid-exposed surface.

Karyopherin Msn5 is involved in a novel mechanism controlling the cellular level of cell cycle regulators Cln2 and Swi5.

The yeast ß-karyopherin Msn5 controls the SBF cell-cycle transcription factor, responsible for the periodic expression of CLN2 cyclin gene at G1/S, and the nuclear export of Cln2 protein. Here we show that Msn5 regulates Cln2 by an additional mechanism. Inactivation of Msn5 causes a severe reduction in the cellular content of Cln2. This occurs by a post-transcriptional mechanism, since CLN2 mRNA level is not importantly affected in asynchronous cultures. Cln2 stability is not significantly altered in msn5 cells and inactivation of Msn5 causes a reduction in protein level even when Cln2 is stabilized. Therefore, the reduced amount of Cln2 in msn5 cells is mainly due not to a higher rate of protein degradation but to a defect in Cln2 synthesis. In fact, analysis of polysome profiles indicated that Msn5 inactivation causes a shift of CLN2 and SWI5 mRNAs from heavy-polysomal to light-polysomal and non-polysomal fractions, supporting a defect in Cln2 and Swi5 protein synthesis in the msn5 mutant. The analysis of truncated versions of Cln2 and of chimeric cyclins combining distinct domains from Cln2 and the related Cln1 cyclin identified an internal region in Cln2 from 181 to 225 residues that when fused to GFP is able to confer Msn5-dependent regulation of protein cellular content. Finally, we showed that a high level of Cln2 is toxic in the absence of Msn5. In summary, we described that Msn5 is required for the proper protein synthesis of specific proteins, introducing a new level of control of cell cycle regulators.

Molecular strategies to increase yeast iron accumulation and resistance.

All eukaryotic organisms rely on iron as an essential micronutrient for life because it participates as a redox-active cofactor in multiple biological processes. However, excess iron can generate reactive oxygen species that damage cellular macromolecules. The low solubility of ferric iron under physiological conditions increases the prevalence of iron deficiency anemia. A common strategy to treat iron deficiency consists of dietary iron supplementation. The baker's yeast Saccharomyces cerevisiae is used as a model eukaryotic organism, but also as a feed supplement. In response to iron deficiency, the yeast Aft1 transcription factor activates cellular iron acquisition. However, when constitutively active, Aft1 inhibits growth probably due to iron toxicity. In this report, we have studied the consequences of using hyperactive AFT1 alleles, including AFT1-1UP, to increase yeast iron accumulation. We first characterized the iron sensitivity of cells expressing different constitutively active AFT1 alleles. We rescued the high iron sensitivity conferred by the AFT1 alleles by deleting the sphingolipid signaling kinase YPK1. We observed that the deletion of YPK1 exerts different effects on iron accumulation depending on the AFT1 allele and the environmental iron. Moreover, we determined that the impairment of the high-affinity iron transport system partially rescues the high iron toxicity of AFT1-1UP-expressing cells. Finally, we observed that AFT1-1UP inhibits oxygen consumption through activation of the RNA-binding protein Cth2. Deletion of CTH2 partially rescues the AFT1-1UP negative respiratory effect. Collectively, these results contribute to understand how the Aft1 transcription factor functions and the multiple consequences derived from its constitutive activation.

Superiority of oxaliplatin and fluorouracil-leucovorin compared with either therapy alone in patients with progressive colorectal cancer after irinotecan and fluorouracil-leucovorin: interim results of a phase III trial.

PURPOSE: In North America, no effective therapy has been available for patients with progressive metastatic colorectal cancer after front-line treatment with irinotecan, bolus fluorouracil (FU), and leucovorin (IFL). PATIENTS AND METHODS: Patients with metastatic colorectal cancer who progressed after IFL therapy were randomly assigned to bolus and infusional FU and leucovorin (LV5FU2), single-agent oxaliplatin, or the combination (FOLFOX4). This planned interim analysis evaluated objective response rate (RR), time to tumor progression (TTP), and alleviation of tumor-related symptoms (TRS) in an initial cohort of patients. RESULTS: Between November 2000 and September 2001, 463 patients from 120 sites in North America were randomly assigned to treatment. FOLFOX4 proved superior to LV5FU2 in all measures of clinical efficacy. Objective RRs determined by an independent radiology panel were 9.9% for FOLFOX4 versus 0% for LV5FU2 (Fisher's exact test, P <.0001). Median TTP was 4.6 months for FOLFOX4 versus 2.7 months for LV5FU2 (two-sided, stratified log-rank test, P <.0001). Relief of TRS occurred in 33% of patients treated with FOLFOX4 versus 12% of patients treated with LVFU2 (chi2 test, P <.001). Single-agent oxaliplatin was not superior to LV5FU2 in any measure of efficacy. Patients treated with FOLFOX4 experienced a higher incidence of clinically significant toxicities than patients treated with LV5FU2, but these toxicities were predictable and did not result in a higher rate of treatment discontinuation or 60-day mortality rate. CONCLUSION: For patients with metastatic colorectal cancer, second-line treatment with FOLFOX4 is superior to treatment with LVFU2 in terms of RR, TTP, and relief of TRS.

Randomized multicenter phase II trial of bolus plus infusional fluorouracil/leucovorin compared with fluorouracil/leucovorin plus oxaliplatin as third-line treatment of patients with advanced colorectal cancer.

PURPOSE: The addition of oxaliplatin to fluorouracil (FU) and leucovorin (LV) improves the outcome of patients with colorectal cancer (CRC). This multicenter study evaluated FU/LV with or without oxaliplatin in patients with metastatic CRC after disease progression on sequential fluoropyrimidine and irinotecan. PATIENTS AND METHODS: Two hundred fourteen patients were randomly assigned to receive LV 200 mg/m2 intravenously (IV) and FU 400 mg/m2 IV bolus, followed by FU 600 mg/m2 IV over 22 hours on days 1 and 2, every 2 weeks (LV5FU2); or LV and FU as described, plus oxaliplatin 85 mg/m2 IV over 2 hours on day 1 of the schedule (FOLFOX4). The primary end point was overall response. RESULTS: Baseline characteristics were similar in the two treatment arms. Objective response (complete + partial) rates for LV5FU2 versus FOLFOX4 were 2% v 13% (P = .0027), respectively. Median time to disease progression was 2.4 v 4.8 months (P < .0001), and median survival was 11.4 v 9.9 months (P = .20) for LV5FU2 and FOLFOX4, respectively. Among the 72 patients who crossed over from LV5FU2 to FOLFOX4, 6% responded. Symptomatic improvement was significantly better for patients in the FOLFOX4 arm (32% v 18% for LV5FU2, P = .05). Grade 3/4 toxicities for LV5FU2 and FOLFOX4 were neutropenia (13% and 42%, respectively), diarrhea (6% and 16%, respectively), and overall neuropathy (0% and 6%, respectively). CONCLUSION: In patients with metastatic CRC, the FOLFOX4 regimen was superior to LV5FU2 with a higher response rate and time to disease progression. FOLFOX4 is an effective regimen even after disease progression on two previous chemotherapy regimens with fluoropyrimidines and irinotecan.

Patients rank toxicity against progression free survival in second-line treatment of advanced renal cell carcinoma.

BACKGROUND: The aims of this study were to quantify and contrast patient preferences between second-line advanced renal cell carcinoma (RCC) medication profiles and their associated benefits and toxicities, and to help frame the doctor-patient discussion about selecting appropriate RCC therapies. RESEARCH DESIGN AND METHODS: Adult residents of the US with a diagnosis of RCC completed a Web-enabled choice-format conjoint survey consisting of a series of 10 treatment-choice questions, each of which included a pair of hypothetical RCC medication profiles. Each profile was described by various medication attributes (features or outcomes) with varying levels. The attributes included efficacy (progression-free survival [PFS]), tolerability (fatigue, stomach problems, mucositis or stomatitis, hand-foot syndrome [HFS]), serious but rare adverse events (pneumonitis, hepatic impairment), and mode of administration. Treatment-choice questions were based on an experimental design with known statistical properties. Random-parameters logit regression was used to estimate relative preference weights for each attribute level. Benefit equivalent measures (additional months of PFS in exchange for toxicities) were also calculated. RESULTS: Of the 272 patients who completed the survey, the majority were female (53%), white (92%), and had at least a college degree (66%). The mean age was 57 years (standard deviation: 10 years). Over the range of attributes and attribute levels included in the survey, PFS was the most important attribute, followed by fatigue, stomach problems, hepatic impairment, mucositis or stomatitis, HFS, pneumonitis, and mode of administration. To reduce severe fatigue to mild-to-moderate fatigue, patients on average would be willing to forego 4.4 months of PFS. To reduce hepatic impairment risk from 0.5% to 0.0%, patients on average would be willing to forego 1.0 month of PFS. The main study limitation was that patients answered hypothetical treatment-choice questions. CONCLUSIONS: This study provides information to physicians about patient priorities when reviewing and selecting RCC therapies with patients.